The present invention relates to an equalizer which is provided in a receiver that is used for digital radio communication, digital mobile radio communication, etc.
FIG. 5 shows one example of the arrangement of conventional equalizers, disclosed, for example, in "Instrumentation and Control" Vol. 25, No. 12 (Dec. 1986), pp. 22-28.
In the figure: reference numeral 1 denotes a received signal input terminal; 10 to 13 delay elements each of which delays by a time T a signal that is inputted through the terminal 1; 20 to 23 weight circuits which multiply the input signal and the signals delayed through the delay elements 10 to 13 by weights (hereinafter referred to as "tap coefficients") a.sub.o (n) to a.sub.N (n), respectively, and output the results; 30 an adder which adds together the outputs of the weight circuits 20 to 23 and outputs the result; 40 an output terminal from which is outputted the result of the addition in the adder 30; 50 a reference signal input terminal from which is inputted a reference signal d(n), which is a known signal sequence; 31 an adder which obtains a difference between the reference signal, i.e., known signal sequence; that is inputted through the terminal 50 and the output from the adder 30; and 60 an error signal output terminal from which is delivered the output of the adder 31 as being an error signal .epsilon.(n). A section in this arrangement which comprises the delay elements 10 to 13, the weight circuits 20 to 23 and the adder 30 is referred to as an equalizing circuit 80.
Examples of equalizer structures include, in addition to the feedforward type equalizer shown in FIG. 5, a feedback type equalizer shown in FIG. 6, a decision feedback type equalizer shown in FIG. 7, and a decision feedback type equalizer that is a combination of the two equalizer structures, shown in FIGS. 5 and 7, i.e., that uses both a feedforward section and a feedback section. In FIGS. 6 and 7, the same reference numerals as those shown in FIG. 5 denote the same elements or portions.
Reference numeral 70 in FIG. 7 denotes a decision element, which, in the case of binary decision, decides between binary data, that is, +1 and -1, by judging to which one of the binary data (+1 and -1) the output of the adder 30 is closer. The decision element 70 defines the result of the decision as an output signal y(n) and also as an input signal to the delay element 10 in the feedback section.
The feedback type equalizer that is shown in FIG. 6 does not employ a decision element, such as that employed in the arrangement shown in FIG. 7, but uses the output of the adder 30 as an input signal to the delay element 10 in the feedback section without making a decision.
FIG. 8 shows the arrangement of packet data that comprises a known signal sequence and a random data sequence for estimation of transmission channel characteristics.
The operation will next be explained.
In an equalizer that has an arrangement such as that shown in FIG. 5, a received signal x(n) (n is a parameter representative of discrete time t=n) that is inputted to the input terminal 1 is divided into two, one of which is inputted to the delay element 10, and the other of which is inputted to the weight circuit 20 that has a tap coefficient a.sub.o (n), where it is weighted and then outputted. Similarly, the output of the delay element 10 is defined as a received signal x(n-1) at t=(n-1), which is then divided into two, one of which is inputted to the delay element 11, and the other of which is inputted to the weight circuit 21 that has a tap coefficient a.sub.1 (n), where it is weighted and then outputted. If this operation is carried out with respect to all of N delay elements and N+1 weight circuits, the output y(n) of the adder 30 at the time n is given by ##EQU1## This is a linear time varying filter, and the z-transform of the transfer function of this time varying filter is given by ##EQU2##
Incidentally, in the case of an unknown transmission channel whose characteristics vary continuously, for example, a fading channel, it is necessary in order to obtain excellent transmission quality to continuously compensate for the distortion caused by introducing an equalizer. That is, the tap coefficient a.sub.1 (n) (i=0, 1, . . . , N) shown in Expressions (1) and (2), which is a function of the parameter n that represents time, must be adaptively controlled so as to optimally compensate for the distorted channel, for each time.
A typical example of this adaptive control employs a reference signal d(n), that is a known signal sequence, which is inputted through the reference signal input terminal 50 in the arrangement shown in FIG. 5. This reference signal is a known signal sequence that is sent in advance of data, as shown in FIG. 8. While this known signal sequence is being sent, the characteristics of the transmission channel are estimated and the tap coefficients are decided to realize ideal transmission characteristics. In general, the error signal .epsilon.(n), which is given by the difference between the reference signal d(n) and the output signal y(n), is defined by EQU .epsilon.(n)=d(n)-y(n) (3)
By using the error signal .epsilon.(n), the tap coefficients are adaptively controlled so that an error is minimized.
Various algorithms, such as those mentioned below, may be used for adaptive control of the tap coefficients according to various purposes. For example, the LMS (Least Mean Square) algorithm is expressed as follows: EQU a(n+1)=a(n)+.mu..epsilon.(n).times.(n) (4)
where .mu. is a parameter which is known as step size parameter, and .epsilon.(n) is an error value that is given by Expression (3).
The Kalman filter algorithm is expressed as follows: ##EQU3## where the superscript T denotes transposition of the matrix.
With an adaptive control algorithm such as those described above, the transmission channel characteristics are estimated and the tap coefficients are determined.
After the transmission channel characteristics are estimated and the tap coefficients are compensatively controlled on the basis of the reference signal, which is a known signal sequence, a random data sequence that is sent after the known signal sequence, as shown in FIG. 8, is subjected to equalization. To effect the equalization, the following two methods may be employed: a first equalization method wherein the tap coefficients of the equalizer that are determined by the reference signal, which is a known signal sequence, are fixed and not updated and, in this state, equalization of the random data section is effected; and another equalization method wherein the output signal y(n) for the random data section is decided by using as initial values the tap coefficients which have been determined by the received signal corresponds to a known signal sequence, and with the result of the decision being regarded as a reference signal d(n), the equalization of the random data section is adaptively effected. With these methods, the equalization of the random data section is conducted.
The conventional equalizers that are arranged as described above involve various problems described below. For example, when the change of transmission channel characteristics is rapid, an equalizer, which estimates the transmission channel characteristics only on the basis of a known signal sequence and effects equalization of the data section with the tap coefficients being fixed, becomes unable to track the rapid change of the characteristics as the equalization progresses toward the trailing end of the data section, resulting in a performance degradation of the received signal. In an equalizer which adaptively equalizes the data section by using as initial values tap coefficients that are set on the basis of a known signal sequence, the amount of computational complexity that is required to update the tap coefficients becomes enormous, which limits the rate at which data can actually be transmitted.